1. Field of the Invention
The invention relates to a fuel cell stack formed from a plurality of stacked unit cells, and more particularly to a technology that favorably enables the fuel cell stack to be started up more easily at extremely low temperatures below freezing point.
2. Description of the Related Art
A fuel cell is formed with a layered construction in which an electrolyte membrane, such as a solid polymer membrane, is sandwiched between respective electrodes and separators provided on each side of the electrolyte membrane. Normally, a plurality of single unit cells with the above construction are stacked and used as a fuel cell stack. Such a fuel cell stack, as for example disclosed in Japanese Patent Laid-Open Publication Nos. 06-060904 and 07-282835, includes respective collector plates provided at each end of the cell units in a stacking direction thereof. The current generated by each cell is drawn from respective output terminal connected to the collector plates.
However, in the case of a vehicle fuel cell stack used in an electric car or the like, the temperature of the unit cells may potentially drop below freezing point in some usage environments, such as in cold regions. At the cathode of each unit cell, a chemical reaction takes places in which water is generated from hydrogen ions that pass through the electrolyte membrane and oxygen within the oxidation gas. However, when the fuel cell stack is started up below freezing point, the generated water freezes and becomes ice, which obstructs the supply of oxygen to the cathode. As a result, there is a reduction in the voltage of the unit cells. However, heat of reaction is generated at the same time as the water in the above described chemical reaction. Accordingly, if the temperature of the unit cells is raised to above freezing point by this heat of reaction, the ice melts and the supply of oxygen to the cathode is restored.
However, the end unit cells positioned to the outer sides of the fuel cell stack are deprived of a substantial amount of heat by the respective collector plates that have large heat capacity and high thermal conductivity. Accordingly, the temperature rise of these end unit cells is slow as compared to the other unit cells. As a result, even if the other unit cells in the fuel cell stack are generating electricity normally, it is possible that the end unit cells will receive insufficient oxygen due to delayed melting of the ice, which in turn may cause the voltage of the end unit cells to be reduced. It is possible that this reduced voltage of the end unit cells will cause the overall output of the fuel cell stack to be reduced as well. Furthermore, if an abnormal chemical reaction is to occur within the end unit cells in this state, it is possible that (a) the structural material of the end unit cells may be exhausted from the fuel cell stack, or (b) that deterioration of the end unit cells may take place.
Various methods for rapidly increasing the temperature of the end unit cells when the fuel cell stack is started up at low temperature can be suggested such as reducing the plate thickness of the collector plates so as to reduce their heat capacity and inhibit heat radiation from the end unit cells. However, simply reducing the plate thickness of the collector plates in this way is liable to cause an increase in electrical resistance when current is drawn, which may in turn cause battery performance to be impaired by a substantial increase in power loss.
The invention has been conceived of in light of the previously described problems, and aims to provide a solution thereto. It is an object of the invention to provide a fuel cell stack that enables the temperature of end unit cells thereof to be increased rapidly when the fuel cell stack is started up at low temperature, without having any detrimental impact upon battery performance during normal operation.